The invention relates to a shaft prosthesis in accordance with the preamble of the independent patent claim.
Shaft prostheses, especially those for the implantation in the femur, are available on the market in numerous different embodiments. With regard to the manner of the anchoring of the prosthesis in the femur, one can distinguish, the shaft prostheses into cemented and cement-free prostheses. In this it is determined in the planning of the operation whether a cemented or cement-free anchoring of the prostheses comes under consideration as a result of the information about the bone, e.g. on the size of the femur and on its condition, which the orthopedist receives from X-ray photographs, etc. In addition the size of the respective prosthesis which is to be implanted is also determined.
During the operation it is then assumed that a given kind of anchoring is carried out, e.g. a cemented anchoring. Prostheses which are suitable for an anchoring of this kind are known for example from DE-B2-25-37 807 or from EP-A-0,672,396. During the operation the femur is first prepared by the orthopedist. Simply expressed, in this situation a cavity is produced into which the cement is then introduced. Subsequently the prosthesis is introduced into the cement and held in place until the cement hardens and the prosthesis is thus fixed. The introduction of the prosthesis into the cement must take place in such a manner that no inclusions of air arise in the cement. This introduction requires a certain skill on the part of the orthopedist since the cement is so constituted by its flow properties at the time point of the introduction into the femur that a flowing back of cement which has once been displaced practically does not take place. This is correspondingly reflected in the design of the shaft--the design of prostheses to be cemented differs considerably from the design of prostheses to be anchored without cement because the anchoring of cemented prostheses takes place in the cement jacket which surrounds the prosthesis, whereas in prostheses to be anchored without cement the anchoring takes place directly in the bone tissue surrounding the prosthesis.
The decision as to whether a prosthesis is anchored with cement or without cement in the implantation is thus made prior to the operation, namely in the pre-operative planning. The decision on the size of the prosthesis to be implanted is also already made prior to the operation. These decisions then also decide how the cavity to be produced by the orthopedist for the reception of the cement must look or which dimensions it must have respectively.
In several cases it turns out during the operation, namely during the preparation of the femur, however, that, for example as a result of the condition of the bone material, it is possible or even desirable to provide a cement-free anchoring although a cemented anchoring had been planned. At this time point however a change in the decision for the cemented prosthesis is no longer possible because for this a new operation planning must take place as a result of the different shaft design of the prostheses to be anchored with cement and without cement.